Homework Help:
Electric force, tension, charge

Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.65 g and a charge of magnitude q. The balls are attracted to each other, and the threads attached to the balls make an angle of 20.0° with the vertical, as shown in the figure below.

I found part (a) 0.00232 N and part (b) 0.00679 N, but I can't seem to figure out part (c)
I used q = sqrt[ (r^2*Ft*sin 20 )/ k ] = 1.93e-8
That answer is not right. Is my formula incorrect? Please help